Find your own style of play, but listen to advice on this forum and try out some different styles of play. I think the best advice so far is to watch the other players well and take notes.

Example of a read saving money:
Recently I've played with a player who is loose-aggressive-passive in his style. He will raise lots of hands, create a bluffing image and with great skill take stacks from players who try to take him out. He also reads bluffs very well. He has been winning alot from others around me (about 200$), but I'm still down only 4.5$ towards him after 500 hands, still waiting for the right hand to call down his bigger bluffs for a major pot. In short - I avoid taking him on until I know I have the right hand and position. This means that I might fold my JJ on the flop towards him, while I would raise another player with the same hand. I would not bluff him all in with A high either.

Example of a read winning money:
This guy seems to hate me after loosing to a JJJ set I caught on the turn after he slowplayed 777. So everytime he sees me he goes for my stack... My read says he will bluff all in very often if he doesn't hit his flush and he chases flush all the time with Axs and Kxs. So I will call his all in on the river even with top pair and I have now taken most of his stack three times

Another good advice I learned from some comment on this forum is to change table. Don't get married to a table. If the players and the position you have on the table isn't good or the action doesn't fit your play then move. It's not a good idea to stay at a loosing table because you want to get even. I sometime play three tables, then after a short while I eliminate the table I don't like and continue the two better ones.

It's in my opinion ok to play 50% of the flops for a shorter time, given the right table, stack, and the right cards, but for me 18-30%. seems to work best. If I like the table I play close to 30%. If I loose too many "medium" hands I tighten up to 18-20% and try to play more safely and concentrated. Too tighten up easy just focus on folding medium hands in early and middle position. Let them connectors go

You mention middle pair players. I think some of my biggest losses have come against someone calling my preflop raise and then again calling a pot bet on the flop with say A9s and a K29 flop against my AK, and then they finally hit a 9 on the turn. This will happen and I think sometimes a middle pair is very much worth playing, given that the oponents stack is big enough if you hit. To avoid this from oponents try to bet higher preflop to get them out. Players have different figures for what they concider a folding preflop bet. I usually bet 4xBB with strong hands, but if I get 5-6 callers on a very loose table I start to bet 6-8-10xBB because someone will call you anyway. And with AA, 5 callers and a QQ9 flop you should fold to any raise

Many people at this forum will tell you to raise more preflop. That advice is good, but you shouldn't start to raise everything to increase your PFR%.
In fact I find it hard to get higher than 4.5-5% PFR even when I raise: AA, KK, QQ, JJ, TT, AKs, AKo, AQs, AQo, AJs and some other hands I only raise from late position. So I find I have to raise medium to bad hands to get higher than 5% PFR and so far this has only cost me money. Improve your aggressive play on the good hand before you start raising the "bad" once.

I'm not a great poker player, played less than a year, but I get along fine now and play almost only 0.25/0.5$ NL 10 player. I started with 200+50$ bonus and bankroll is now 1400$ - including a bad beat jackpot of 275$ though.

Good luck in the ring